Bipolar separator for a fuel cell

ABSTRACT

The invention relates to a bipolar separator ( 17 ) including a first ( 33 ) and a second ( 35 ) polar plate each comprising an inner surface and an outer surface in which at least one distribution channel ( 53, 55 ) is formed, the channels formed in the outer surfaces of the first and the second polar plate enabling fuel and oxidizer, respectively, to flow. The bipolar separator further includes an inner layer ( 29 ) provided to be sandwiched and compressed between the substantially planar inner surfaces of the first ( 33 ) and the second ( 35 ) polar plate, so as to form a laminated structure. The inner layer is formed by a perforated sheet comprising a group of through-grooves that form branchless channels, the ends of which lead, respectively, to two manifolds such that the coolant is able to flow between the first ( 33 ) and the second ( 35 ) polar plate.

FIELD OF THE INVENTION

The present invention generally relates to fuel cells. More specifically, it relates to a bipolar separator arranged to impermeably separate two contiguous electrochemical cells of a fuel cell.

PRIOR ART

A fuel cell stack is a stack of electrochemical fuel cells electrically connected in series. When they are supplied with a fuel stream and with an oxidant stream, the electrochemical cells of a stack are the setting for a controlled chemical reaction that generates electric power, as well as reaction products and heat. The cells of a fuel cell generally comprise a polymer electrolyte membrane (PEM) arranged between two porous electrodes (a cathode and an anode). The assembly comprising the PEM, the anode and the cathode, the first being sandwiched between the latter two, forms what is referred to as a membrane-electrode assembly (MEA). In order to promote the desired electrochemical reaction, the anode and the cathode of the MEA generally comprise one or more catalysts.

An electrochemical fuel cell operates on the basis of the following principle. The cell is supplied with a fuel stream via the anode side. By passing through the porous electrode layer, the fuel is oxidised by the attendant catalyst(s), causing the formation of cations (generally protons) and electrons. The cations can migrate through the electrolyte membrane towards the cathode. At the same time, an oxidant stream (generally pure oxygen or a mixture containing oxygen) is supplied via the cathode side. The oxidant moves through the porous electrode and reacts with the cations that have passed through the electrolyte membrane. For their part, the electrons pass from the anode to the cathode through an external conductor, which generates an electric current. The electrochemical reaction between the fuel and the oxidant also generates heat. This heat tends to increase the temperature of the fuel cell stack when it operates.

In order to obtain a significant amount of electric power, stacking a certain number of electrochemical cells is known. In this case, a separation wall made of an electrically conductive material is interposed between each cell so that said cells are connected in series. Such a stack of electrochemical cells is called a fuel cell stack and the separation walls between the cells are called bipolar separators. For applications that require considerable power, such as electrical mobility, for example, very large fuel cell stacks are often used. These stacks sometimes can comprise a hundred or more electrochemical cells. As can be expected, such fuel cell stacks generate a lot of heat and they must be actively cooled when they are operated.

A regular fuel cell therefore is basically formed by a stack, within which MEAs and bipolar separators alternate. Thus, a bipolar separator is normally always interposed between two membrane-electrode assemblies and, conversely, a membrane-electrode assembly is normally always sandwiched between two bipolar separators. In addition to sealing neighbouring fuel cells from each other, the bipolar separators are generally designed to allow the fuel cell stack to be supplied and cooled. To this end, each bipolar separator generally comprises three types of ducts: ducts used for the passage of the fuel gas supplied to the anode, ducts used for the passage of the gaseous oxidant supplied to the cathode and, finally, ducts used for the passage of the cooling liquid.

In most cases, the bipolar separators are formed by two thin, electrically conductive (mono)polar plates (an anode plate and a cathode plate). These two polar plates are arranged back-to-back and are connected by means such as solder or adhesive. The outer face of each polar plate comprises distribution channels. The distribution channels of the anode plate form the ducts for the fuel supply and those of the cathode plate form the ducts for the oxidant supply. A known economical method for producing polar plates corresponding to the above description is to start from flat metal sheets and to form the distribution channels in the surface of the sheets by stamping.

With such a stamping method, the inner face of the polar plates is not flat, but has raised reliefs that are the negative image of the hollow reliefs that form the channels of the outer face. It will be appreciated that the raised reliefs on the inner faces of the two polar plates prevent their juxtaposition. Thus, a bipolar separator formed by assembling and connecting a pair of stamped sheets normally comprises a network of interstitial spaces between the two polar plates. In a manner per se known, the voids between the two plates can form a flow field for the cooling liquid.

A problem with the aforementioned known solution is that the configuration and the dimensions of the network of interstitial spaces for the passage of the cooling liquid are dependent, both on the configuration and on the dimensions of the fuel distribution channels and on those of the oxidant distribution channels. As a result, most of the time the geometry of the passage arranged in the bipolar separators for the cooling liquid is by no means optimal.

In order to attempt to overcome this problem, interposing spacers between the two polar plates has been proposed. The spacers allow both the presence of non-cooled zones and excessive pressure losses in the cooling ducts to be avoided. However, a disadvantage of the latter solution is that it results in a considerable increase in the flow section for the cooling liquid between the two plates. It will be appreciated that such an increase has the disadvantage of being accompanied by a concomitant increase in the volume of liquid in the system for cooling the fuel cell. The weight of the liquid, and thus the weight of the cooling system as a whole, is thus increased.

BRIEF DISCLOSURE OF THE INVENTION

An aim of the present invention is to overcome the aforementioned disadvantages of the prior art. The present invention achieves this aim by supplying a bipolar separator according to appended claim 1.

An advantage of the bipolar separator according to the invention is that its construction allows the flow and the volume occupied by each of the fluids passing through the bipolar separator (namely the fuel, the oxidant and the cooling liquid) to be independently optimised.

According to the invention, the flow field formed between the inner faces of the first and the second polar plate is formed by a group of through-grooves formed in a perforated sheet that is sandwiched between the substantially flat inner faces of the first and the second polar plate. By virtue of this feature, it is possible to determine the section and the course of the path to be covered by the cooling liquid, simply by adjusting the thickness of the perforated sheet and the geometry of the grooves. A second advantage of the invention therefore is to allow the volume of liquid to be considerably reduced in the system for cooling the fuel cell, simply by optimising the configuration and the dimensions of the inner channels, which allow the cooling liquid to flow between the polar plates.

According to an advantageous embodiment of the invention, the bipolar separator is in the shape of a right-angled quadrilateral (a square or a rectangle). According to this embodiment, the two manifolds of the pair of first manifolds and the two manifolds of the pair of second manifolds are arranged along the same two opposite sides of the right-angled quadrilateral. The two manifolds of the pair of third manifolds, for their part, are arranged along the other two opposite sides of the right-angled quadrilateral.

According to a preferred variation of the aforementioned embodiment, the bipolar separator comprises at least two pairs of third manifolds, each pair of third manifolds comprising a third supply manifold and a third discharge manifold, the manifolds of the various pairs of third manifolds being arranged side-by-side, along the same two opposite sides of the rectangle.

According to this variation, each pair of third manifolds is associated with a group of channels that extends in the plane of the inner layer between the two manifolds of the pair. The channels of the group substantially extend in straight lines between two opposite sides, so that the part of the perforated sheet that is located facing the active zone 51 of the MEA is divided into a plurality of flow fields that are separate and parallel to each other, each flow field being formed by the group of channels associated with one of the pairs of third manifolds. It will be appreciated that this arrangement allows the length of the path covered by the cooling liquid inside the bipolar separator to be shortened as much as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will become apparent upon reading the following description, which is provided solely by way of a non-limiting example, and with reference to the accompanying drawings, in which:

FIG. 1 is a highly simplified block diagram of a fuel cell system comprising a cooling circuit;

FIG. 2 is an exploded view of a bipolar separator according to a first specific embodiment of the invention;

FIG. 3 is a perspective view of the bipolar separator of FIG. 2, with its various constituent elements being shown assembled;

FIG. 4 is a partial view of the separator of FIGS. 2 and 3, shown as a section view along A-A of FIG. 3;

FIG. 5 is in an enlarged view of part of the section of FIG. 4;

FIG. 6 is a perspective view of the perforated sheet forming the inner layer of a bipolar separator according to a second specific embodiment of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

FIG. 1 is a highly simplified block diagram of a fuel cell system 1 that particularly comprises a cooling circuit 10. By way of an example, the system 1 can be installed in a vehicle, represented by the rectangle 2. The core of the fuel cell system is the fuel cell 3, which itself is formed by a stack of fuel cells. The fuel cells (only one of which is shown in FIG. 1) each contain a polymer electrolyte membrane (PEM) arranged between two porous electrodes. According to this figure, each fuel cell is divided into three compartments. An anode compartment 4, through which the fuel gas passes, a cathode compartment 5, through which the gaseous oxidant passes and, finally, a compartment 6, through which the cooling liquid passes. The three compartments 4, 5 and 6 of a fuel cell are respectively connected to the three compartments of each of the other cells (not shown) of the fuel cell 3 by means of manifolds (not shown). Conventionally, the cathode compartment 5 is supplied with oxidant (for example, oxygen contained in the ambient air) by means of a compressor 7. The anode compartment 4 is supplied with fuel (for example, hydrogen) from a compressed gas tank 8 via a pressure regulator 9 for regulating the pressure. Any fuel that has not been consumed and any air depleted of oxygen then exits the fuel cell via suitable ducts (not shown). In a known manner, these ducts can, for example, take the form of two recirculation loops that respectively return the fuel and the oxidant to the anode and cathode compartments.

The cooling circuit 10 of the system 1 is arranged to cool the fuel cell 3. It will be appreciated that it possibly can be arranged to also cool other elements of the system 1 or, more generally, other elements of the vehicle 2. In the example shown, the cooling liquid that circulates in the circuit 10 typically can be a mixture of water and antifreeze. The cooling liquid is driven by a pump 11 in order to circulate as a loop in the circuit 10. As is also shown in the figure, the pump 11 itself is activated by an electric motor 14. The mixture of water and antifreeze is cooled each time it passes through the heat exchanger 12 (shown in the form of a vehicle radiator associated with a fan 13). The cooling circuit also comprises two thermometers 15, which are arranged at the input of the fuel cell 3 and of the heat exchanger 12, respectively. After passing through the heat exchanger, the cooling liquid enters the compartment 6 of the fuel cell 3. After passing through the fuel cell, the cooling liquid exits it while carrying some of the heat generated by the cell. This heat is then discharged into the atmosphere during a further passage through the heat exchanger 12.

FIG. 2 is an exploded view showing the various layers of a bipolar separator 17 according to a first specific embodiment of the invention. As shown in the figure, the laminated bipolar separator of this example comprises five layers. These layers are, on the one hand, two substantially flat sealing plates (21 and 23) made of an electrically conductive and impermeable material and, on the other hand, three perforated sheets (25, 27 and 29) made of electrically conductive material.

FIG. 2 also shows a Membrane-Electrode Assembly (MEA) (31). Even though the figures do not show any of the constituent elements of the MEA, a person skilled in the art knows that said MEA is itself a laminated structure formed by a polymer electrolyte membrane sandwiched between an anode and a cathode. The specific features of the MEA nevertheless do not directly relate to the invention. In order to facilitate understanding of the following, FIG. 2 delimits, simply by means of a rectangle, the active zone 51 occupied by the centre of the MEA. It will be appreciated that the active zone 51 is the zone in which the electrochemical reaction occurs. By convention, it is also understood that FIG. 2 shows the MEA 31 turned so that the anode is on top.

With reference to FIGS. 2 and 3, it can be seen that the periphery of the bipolar separator 17 is perforated with a plurality of holes that pass through the separator on either side. It will be appreciated that these various holes that pass through the five layers of the separator, as well as the periphery of the MEA, are the constituent elements of ducts arranged to extend in the stacking direction of the fuel cell (not shown), once it is assembled. In a manner per se known, these ducts comprise first manifolds 37, 39, respectively fuel supply and discharge manifolds, second manifolds 41, 43, respectively oxidant supply and discharge manifolds, and third manifolds 45A and 45B, 47A and 47B, respectively cooling liquid supply and discharge manifolds.

Still with reference to FIG. 2, it can be seen that the bipolar separator that is shown is of rectangular shape and that the first manifolds 37, 39 and the second manifolds 41, 43 are arranged on the large sides of the rectangle. Moreover, according to an advantageous embodiment of the invention, the third manifolds are duplicated. In other words, there are two third supply manifolds 45A and 45B and two third discharge manifolds 47A, 47B. It can be seen that the four third manifolds are arranged on the small sides of the rectangle. However, it will be appreciated that, according to an alternative variation of the same embodiment, the first and second manifolds could be arranged on the small sides of the rectangle and the four third manifolds could be arranged on the large sides.

Still with reference to the same figures, it can be seen that the manifolds 37 and 39, respectively the fuel supply and discharge manifolds, each have a constricted passage at the level of the perforated sheet 25. More specifically, it will be noted that the two locations, which are occupied by the openings 37 and 39 in the other sheets, are each occupied by two smaller openings (respectively 371, 372 and 391, 392) in the sheet 25. It is noteworthy from the above that the openings of the fuel supply and discharge manifolds extend towards the inside of the sheet 25 by considerably less distance than is the case in the other sheets. Similarly, it can be seen that the manifolds 41 and 43, respectively the oxidant supply and discharge manifolds, each have a constricted passage at the level of the perforated sheet 27, the openings of the oxidant supply and discharge manifolds extending towards the inside of the sheet 27 by considerably less distance than is the case in the other sheets. Finally, it also can be seen that the manifolds 45A, 45B and 47A, 47B, respectively the cooling liquid supply and discharge manifolds, each have a constricted passage at the level of the perforated sheet 29. Indeed, as has already been explained in relation to the perforated sheets 25 and 27, the locations occupied by the openings 45A, 45B, 47A and 47B in the other sheets are each occupied in the sheet 29 by two smaller openings (respectively 451A and 452A, 451B and 452B, 471A and 472A, 471B and 472B). The result is that the openings of the cooling liquid supply and discharge manifolds extend towards the inside of the sheet 29 by considerably less distance than is the case in the other sheets.

With more particular reference to FIG. 2, it also can be seen that the perforated sheet 25 comprises a plurality of distribution channels (one of these channels is referenced 53). In the embodiment shown, these channels are in the form of grooves that go all the way through the thickness of the sheet 25 (in other words, the grooves are through-grooves). The distribution channels each extend longitudinally between a first end located on the periphery of the perforated sheet, close to the fuel supply manifold, and a second end also located on the periphery, but close to the fuel discharge manifold. In particular, FIG. 2 shows that one of the ends of the channel 53 is located in the vicinity of the opening 371 and that the other end is located in the vicinity of the opening 391. In the example shown, the distribution channels of the sheet 25 extend parallel to each other and they have courses that form U-shapes that are nested together. An advantage of this particular configuration is that the distribution channels are all substantially the same length. FIG. 2 also shows that the parallel channels of the perforated sheet 25 are separated from each other by thin strips of conductive material (one of which is referenced 61). It also can be seen that these thin parallel strips are connected to the rest of the sheet 25 via their two ends.

In a similar manner to that explained above in relation to the perforated sheet 25, the perforated sheet 27 comprises a plurality of distribution channels (one of these channels is referenced 55). In the embodiment shown, these channels are in the form of grooves that go all the way through the thickness of the sheet 27. The distribution channels each extend longitudinally between a first end located on the periphery of the perforated sheet, close to the oxidant supply manifold, and a second end also located on the periphery, but close to the oxidant discharge manifold. It can be seen that, like those of the sheet 25, the distribution channels of the sheet 27 extend parallel to each other, and that they have courses that form U-shapes that are nested together. FIG. 2 also shows that the parallel channels of the perforated sheet 27 are separated from each other by thin strips of conductive material (one of which is referenced 63). It also can be seen that these thin parallel strips are connected to the rest of the sheet 27 via their two ends.

According to the present invention, the perforated sheet that forms the inner layer 29 further comprises a plurality of grooves that go all the way through the thickness of the sheet 29 (one of these grooves is referenced 57). The grooves are arranged to form channels to allow the cooling liquid to pass through the bipolar separator 17. The ducts each extend longitudinally between a first end located on the periphery of the perforated sheet, close to one of the cooling liquid supply manifolds, and a second end also located on the periphery, but close to one of the cooling liquid discharge manifolds. It can be seen that the ducts of the sheet 29 extend parallel to each other substantially in straight lines, and that they are all approximately the same length. FIG. 2 also shows that the parallel channels of the perforated sheet 29 are separated from each other by thin strips of conductive material (one of which is referenced 65). It also can be seen that these thin straight strips are connected to the rest of the sheet 29 via their two ends.

By way of a reminder, an aim of the invention is to reduce the volume of liquid in the cooling circuit and, more specifically, to reduce the volume of cooling liquid present in the bipolar separators. To this end, the applicant has observed that, in order to reduce the volume of cooling liquid in a bipolar separator, it is generally worthwhile attempting to do so without comparably changing the magnitude of the flow of liquid through the separator. This means that, if the intention is to reduce the volume of cooling liquid, it is worthwhile to reduce, substantially by the same proportion, the time taken by the cooling liquid to pass through the separator. One way of reducing the transit time of the liquid inside the separator would naturally be to increase its flow speed. However, a higher flow speed generates turbulence and requires the use of a more powerful pump. To address this issue, the applicant has contemplated the possibility of reducing the length of the ducts that allow the cooling liquid to pass through the separator. Indeed, by shortening the ducts, it is possible to simultaneously reduce the volume thereof and the time taken by the cooling liquid to pass through the separator, without needing to increase the flow speed.

According to an advantageous variation of the invention, the aforementioned possibility is implemented by virtue of the use of substantially straight ducts for the cooling liquid. As shown in FIG. 2, the network of channels of the sheet 29 allows substantially all the active zone 51 of the MEA to be covered using channels that extend substantially in straight lines parallel to each other. It will be appreciated that this arrangement is made possible through the use of at least two pairs of third manifolds, each pair of third manifolds comprising a third supply manifold and a third discharge manifold. Indeed, it can be seen that in one half of the sheet 29, a group of channels extends between a first pair of third manifolds (45A and 47A), whereas in the other half of the sheet 29, a second group of channels extends between the other pair of third manifolds (45B and 47B).

With further reference to FIGS. 2 and 3, it also can be seen that, in the example shown, the bipolar separator 17 is perforated with a circular hole at each of its four corners. In a manner per se known, these four holes 49 are designed to each receive a tension rod, not shown, to longitudinally compress the fuel cell stack.

Still with reference to the same figures, it can be appreciated that, once assembled, the sealing plate 21 and the perforated sheet 25 together form a first polar plate, or anode polar plate 33, with an outer face turned towards the anode of the MEA, and a flat inner face (turned upwards) formed by the back of the sealing plate 21. In much the same way, the sealing plate 23 and the perforated sheet 27 together form a second polar plate, or cathode polar plate 35, with an outer face (turned upwards), in which the oxidant distribution channels are formed, and a flat inner face (turned downwards) formed by the back of the sealing plate 23.

According to the invention, the bipolar separator 17 is a laminated structure formed by assembling the inner layer 29 and the first and the second polar plate 33 and 35. It will be appreciated that the perforated sheet forming the inner layer is sandwiched and compressed between the flat inner faces of the first and the second polar plate. Once the bipolar separator 17 is assembled, the grooves of the inner layer form closed ducts that allow the cooling liquid to flow between the first and the second polar plate.

FIGS. 4 and 5 show, as a section view along A-A of FIG. 3, the cooling liquid supply manifold that is referenced 45B in FIG. 3. The section view of FIG. 4 shows one of the ends 59 of five of the grooves that go all the way through the thickness of the sheet 29. It also can be noted that one of these five grooves is the groove 57 already mentioned with reference to FIG. 2. Reference will now be more particularly made to FIG. 4, which shows that the illustrated grooves all stop before reaching the openings 451B and 452B of the manifold 45B. As a result, the sheet 29 has a list 69 that extends between the ends 59 of the channels and the two openings 451B and 452B. It can be seen that the list 69 connects together the ends of the thin strips of material that extend between the channels (one of these thin ends is referenced 65). It thus will be appreciated that the list 69 ensures the connectivity, and thus the structural integrity, of the perforated sheet 29.

As shown in the two aforementioned figures, the list 69 forms part of a zone of the inner layer 29 that forms a kind of diaphragm partially obstructing the cooling liquid supply manifold 45B. It also will be appreciated that three other zones of the inner layer 29 constrict the manifolds 45A, 47A and 47B in the same way.

According to the embodiment of the invention that is the subject of the present example, the first ends 59 of five grooves are located in the zone of the inner layer 29 that forms a kind of diaphragm inside the manifold 45B. Furthermore, the other ends of grooves of the perforated sheet 29 are, similarly, each located inside one of the manifolds 45B, 45A, 47A and 47B. Therefore, it will be appreciated that the ducts for the passage of the cooling liquid each have a first end that opens into a cooling liquid supply manifold and a second end that opens into a cooling liquid discharge manifold.

As previously stated, the two sealing plates 21 and 23 are made of an impermeable and electrically conductive material. The three perforated sheets 25, 27 and 29 are also made of an electrically conductive material. However, the perforated sheets are not necessarily impermeable. The five layers forming the bipolar separator are connected as a single laminated structure by virtue of the effect of the compression that is axially exerted on the fuel cell. According to a variation, the five layers forming the bipolar separator are connected by the combined effect of the compression and the use of a conductive adhesive. The material from which the various layers of the separator are made can be a metal, but it also can be a composite electrically conductive material. Irrespective of the material that is used, it is advantageous for it to have a Young's modulus, measured perpendicular to the plane of the sheets, that is greater than 240 and preferably is greater than 350.

FIG. 6 is a perspective view of the perforated sheet forming the inner layer of a bipolar separator according to a second specific embodiment of the invention. As in the first embodiment, the perforated sheet 529 comprises a plurality of grooves that go all the way through the thickness of the sheet (one of these grooves is referenced 557). It can be seen that the grooves each extend longitudinally between a first end located on the periphery of the perforated sheet, close to one of the cooling liquid supply manifolds, and a second end also located on the periphery, but close to one of the cooling liquid discharge manifolds. FIG. 6 also shows that the substantially parallel channels of the perforated sheet 529 are separated from each other by thin strips of conductive material (one of which is referenced 565). It also can be seen that these generally straight thin strips are connected to the rest of the sheet via their two ends.

Still with reference to FIG. 6, it can be seen that the inner layer of the bipolar separator is of rectangular shape and that the first manifolds 537, 539 and the second manifolds 541, 543 are arranged on the small sides of the rectangle. Moreover, in the embodiment that is the subject of the present example, as for the previous example, there are two third supply manifolds and two third discharge manifolds. Furthermore, as has already been explained in relation to the first embodiment, the sheet 529 partially obstructs each of the third manifolds. Indeed, it will be appreciated that the locations corresponding to each of the third manifolds are each occupied in the sheet 529 by two openings with a smaller section (551A and 552A, 551B and 552B, 571A and 572A, 571B and 572B).

As mentioned above, if the intention is to reduce the volume of cooling liquid contained in a bipolar separator, then substantially reducing, and even preferably proportionately reducing, the length of the ducts allowing the passage of the cooling liquid is advantageous. Consequently, the embodiment of FIG. 6 corresponds to the preferred embodiment. Indeed, by simultaneously referring to FIGS. 2 and 6, it can be seen that the perforated sheet 29 and the perforated sheet 529 are basically set apart from each other through a different orientation of the parallel grooves forming the channels. Indeed, according to the embodiment shown in FIG. 6, the length of the grooves is particularly reduced as they extend parallel to the small side of the rectangle. Furthermore, it can be seen that the grooves form four groups of channels that respectively extend between the opening 551A and the opening 571A, between the opening 552A and the opening 572A, between the opening 551B and the opening 571B and, finally, between the opening 552B and the opening 572B.

It also will be appreciated that various modifications and/or improvements that are obvious to a person skilled in the art can be made to the embodiment that is the subject of the present description without departing from the scope of the present invention that is defined by the appended claims. In particular, the anode polar plate and the cathode polar plate do not necessarily have a laminated structure. Indeed, alternatively it can involve a monolithic polar plate comprising an outer face, in which at least one distribution channel is formed, and a substantially flat inner face. 

1. Bipolar separator (17) for a fuel cell stack (3) comprising: a first (33) and a second (35) impermeable polar plate made of electrically conductive material, each of the polar plates comprising an outer face, in which at least one distribution channel (53, 55) is formed, and an inner face, the channels formed in the outer faces of the first and the second polar plate respectively allowing fuel and oxidant to circulate; a flow field formed between the inner faces of the first (33) and the second (35) polar plate allowing a cooling liquid to circulate; a pair of first manifolds (37, 39), a pair of second manifolds (41, 43) and a pair of third manifolds (45A, 45B, 47A, 47B) extending in the stacking direction of the fuel cell stack, the manifolds of the pair of first manifolds being respectively arranged to supply and to discharge the fuel intended to circulate in the distribution channel (53) of the first polar plate (33), the manifolds of the pair of second manifolds being respectively arranged to supply and to discharge the oxidant intended to circulate in the distribution channel (55) of the second polar plate (35) and the manifolds of the pair of third manifolds being respectively arranged to supply and to discharge the cooling liquid intended to circulate in the flow field, wherein the bipolar separator comprises an inner layer (29) formed by a perforated sheet made of an electrically conductive material, the inner layer being designed to be sandwiched and compressed between the substantially flat inner faces of the first (33) and the second (35) polar plate, so as to form a laminated structure, the perforated sheet comprising a group of through-grooves that form branchless channels, the ends of which respectively emerge into the two manifolds of the pair of third manifolds, so as to allow the cooling liquid to circulate between the first (33) and the second (35) polar plate.
 2. Bipolar separator for a fuel cell stack (3) according to claim 1, wherein the bipolar separator it is in the shape of a right-angled quadrilateral, the first manifolds (37, 39) and the second manifolds (41, 43) being arranged along the same two opposite sides of the right-angled quadrilateral, and wherein the bipolar separator comprises at least two pairs of third manifolds (45A, 47A, 45B, 47B), each pair of third manifolds comprising a third supply manifold and a third discharge manifold arranged facing each other along the other two opposite sides of the right-angled quadrilateral.
 3. Bipolar separator for a fuel cell stack (3) according to claim 2, wherein the at least two pairs of third manifolds (45A, 47A, 45B, 47B) are each associated with a group of channels (57) extending in the plane of the inner layer (29) between the two manifolds of one of the pairs of third manifolds, the channels of said group substantially extending in straight lines and perpendicular to said other opposite sides of the right-angled quadrilateral, so that the part of the inner layer that is located facing the active zone 51 of the MEA is divided into a plurality of flow fields that are separate and parallel to each other, each flow field being formed by the group of channels associated with one of the pairs of third manifolds.
 4. Bipolar separator for a fuel cell stack (3) according to claim 3, wherein two distinct zones of the inner sheet (29) respectively partly obstruct the two manifolds of each pair of third manifolds (45A, 45B, 47A, 47B), so that the third manifolds each have a constricted passage at the level of the inner layer (29), and wherein the two ends of the channels of the group of channels associated with each pair of third manifolds are respectively located inside said two separate zones that partly obstruct the two manifolds of the corresponding pair of third manifolds, so that the ends of the channels respectively open into the manifolds of the corresponding pair of third manifolds.
 5. Bipolar separator for a fuel cell stack (3) according to claim 3, wherein the bipolar separator is square shaped.
 6. Bipolar separator for a fuel cell stack (3) according to claim 3, wherein the bipolar separator is rectangular shaped, in wherein said same two opposite sides are the small sides of the rectangle, and wherein said other two opposite sides are the large sides of the rectangle.
 7. Bipolar separator for a fuel cell stack (3) according to claim 4, wherein said two separate zones that partly obstruct the two manifolds of each pair of third manifolds each comprise an list separating the ends of the channels from the constricted passages of the corresponding pair of third manifolds.
 8. Bipolar separator for a fuel cell stack (3) according to claim 1, wherein the first (33) or the second (35) polar plate comprises: a substantially flat and impermeable sealing plate (21 or 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25 or 27) made of electrically conductive material, the outer layer comprising at least one perforation; the sealing plate (21 or 23) and the outer layer (25 or 27) being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel (53 or 55) formed in the outer face of said first (33) or second (35) polar plate.
 9. Bipolar separator for a fuel cell stack (3) according to claim 1, wherein the first (33) and the second (35) polar plate each comprise: a substantially flat and impermeable sealing plate (21, 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25, 27) made of electrically conductive material, the outer layer comprising at least one perforation (53, 55); the sealing plate and the outer layer being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel formed in the outer face of each of the two polar plates (33, 35).
 10. Bipolar separator for a fuel cell stack (3) according to claim 4, wherein the bipolar separator is square shaped.
 11. Bipolar separator for a fuel cell stack (3) according to claim 4, wherein the bipolar separator is rectangular shaped, in wherein said same two opposite sides are the small sides of the rectangle, and wherein said other two opposite sides are the large sides of the rectangle.
 12. Bipolar separator for a fuel cell stack (3) according to claim 2, wherein the first (33) or the second (35) polar plate comprises: a substantially flat and impermeable sealing plate (21 or 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25 or 27) made of electrically conductive material, the outer layer comprising at least one perforation; the sealing plate (21 or 23) and the outer layer (25 or 27) being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel (53 or 55) formed in the outer face of said first (33) or second (35) polar plate.
 13. Bipolar separator for a fuel cell stack (3) according to claim 3, wherein the first (33) or the second (35) polar plate comprises: a substantially flat and impermeable sealing plate (21 or 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25 or 27) made of electrically conductive material, the outer layer comprising at least one perforation; the sealing plate (21 or 23) and the outer layer (25 or 27) being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel (53 or 55) formed in the outer face of said first (33) or second (35) polar plate.
 14. Bipolar separator for a fuel cell stack (3) according to claim 4, wherein the first (33) or the second (35) polar plate comprises: a substantially flat and impermeable sealing plate (21 or 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25 or 27) made of electrically conductive material, the outer layer comprising at least one perforation; the sealing plate (21 or 23) and the outer layer (25 or 27) being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel (53 or 55) formed in the outer face of said first (33) or second (35) polar plate.
 15. Bipolar separator for a fuel cell stack (3) according to claim 5, wherein the first (33) or the second (35) polar plate comprises: a substantially flat and impermeable sealing plate (21 or 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25 or 27) made of electrically conductive material, the outer layer comprising at least one perforation; the sealing plate (21 or 23) and the outer layer (25 or 27) being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel (53 or 55) formed in the outer face of said first (33) or second (35) polar plate.
 16. Bipolar separator for a fuel cell stack (3) according to claim 6, wherein the first (33) or the second (35) polar plate comprises: a substantially flat and impermeable sealing plate (21 or 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25 or 27) made of electrically conductive material, the outer layer comprising at least one perforation; the sealing plate (21 or 23) and the outer layer (25 or 27) being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel (53 or 55) formed in the outer face of said first (33) or second (35) polar plate.
 17. Bipolar separator for a fuel cell stack (3) according to claim 7, wherein the first (33) or the second (35) polar plate comprises: a substantially flat and impermeable sealing plate (21 or 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25 or 27) made of electrically conductive material, the outer layer comprising at least one perforation; the sealing plate (21 or 23) and the outer layer (25 or 27) being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel (53 or 55) formed in the outer face of said first (33) or second (35) polar plate.
 18. Bipolar separator for a fuel cell stack (3) according to claim 2, wherein the first (33) and the second (35) polar plate each comprise: a substantially flat and impermeable sealing plate (21, 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25, 27) made of electrically conductive material, the outer layer comprising at least one perforation (53, 55); the sealing plate and the outer layer being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel formed in the outer face of each of the two polar plates (33, 35).
 19. Bipolar separator for a fuel cell stack (3) according to claim 3, wherein the first (33) and the second (35) polar plate each comprise: a substantially flat and impermeable sealing plate (21, 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25, 27) made of electrically conductive material, the outer layer comprising at least one perforation (53, 55); the sealing plate and the outer layer being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel formed in the outer face of each of the two polar plates (33, 35).
 20. Bipolar separator for a fuel cell stack (3) according to claim 4, wherein the first (33) and the second (35) polar plate each comprise: a substantially flat and impermeable sealing plate (21, 23) made of electrically conductive material; an outer layer formed by a perforated sheet (25, 27) made of electrically conductive material, the outer layer comprising at least one perforation (53, 55); the sealing plate and the outer layer being adjoined so as to form a laminated structure, the perforation of the outer layer forming the channel formed in the outer face of each of the two polar plates (33, 35). 